Heredity, environment, nervous system

Heredity and environment

Every human action, behavior, thought, or tendency is shaped by a combination of what you inherit from your ancestors (heredity) and what you experience through life (environment). This is often described as “nature and nurture.” Understanding how these two influences work together is essential in psychological science because it helps explain human reactions, behavior, and emotions.

Heredity (“nature”)

Heredity provides a set of genes (encoded within DNA) passed from parents to children that influences physical traits, mental capabilities, temperament, and susceptibility to disorders.

For example, research suggests that intellect, typical emotional responses, and cognitive tendencies are each partly shaped by genes. If two parents have strong reasoning skills, their child may have higher odds of developing similar abilities. At the same time, it’s important to understand what genes do and don’t do: the genome can set a range of possible pathways, but it doesn’t lock in a single outcome. Heredity offers possibilities, and the environment influences which of those possibilities become real.

Even genes with strong effects don’t guarantee that a trait will appear. Someone who carries genetic markers associated with pervasive worry may never develop those patterns if their environment includes protective factors (such as supportive relationships or learning skills for emotional stability). In one context, a gene might contribute to distress; in another, it might have little visible effect. This shows that biological inheritance is a guide, not a sentence.

In summary, what does heredity contribute to psychology’s understanding of human behavior?

Environmental factors (“nurture”)

Your environment includes all influences encountered after conception (such as nutrition, family upbringing, education, and social relationships). The environment affects how, when, and whether inherited potentials show up.

Suppose a child has musicians in their family. If that child grows up with access to instruments, skilled teachers, and a home filled with music, their abilities may develop strongly. In contrast, the same child in an unsupportive environment may never show that inherited potential. The reverse can also happen: a person without an obvious early musical advantage may become highly skilled through practice and encouragement. This illustrates how powerful environmental conditions can be.

Across many areas of life, environmental influences shape how people speak, manage emotions, perform in school, and develop values. Even siblings raised in the same home can become very different. Those differences may come from subtle differences in treatment, different peer groups, timing, or outside events. These contrasts highlight the environment’s ability to refine human capacities - sometimes compensating for genetic predispositions and other times amplifying them.

Evolution and psychology

To understand why certain behaviors persist, researchers use the concept of natural selection. This idea proposes that traits that increase the chances of survival or reproduction become more common over many generations. For example, quick caution around heights or crawling creatures may seem unnecessary today, but it likely helped ancestors avoid injury or predators, increasing their chances of surviving and having offspring.

Some behaviors that were useful in earlier environments - such as rapid fear responses or heightened alertness - may now show up as problems like persistent anxiety or strong reactions to minor risks. Similarly, social tendencies such as conformity or competitiveness may have supported group cohesion or access to resources in the past, but they can be less adaptive in modern settings.

The evolutionary perspective is meant to explain the origins of behavior, not excuse it. Evolutionary explanations should be evaluated carefully, because misuses of these ideas have been used to justify discrimination or promote social divisions (such as in eugenics).

Untangling genetics and experience: research strategies

Psychologists use several research designs to separate genetic influences from environmental influences:

• Twin research: Comparing identical twins (who share essentially all of their DNA) with fraternal twins (who share about half their genes on average) helps estimate how heritable a trait is. If identical twins are more similar to each other on a trait than fraternal twins are - and identical twins raised apart remain highly similar despite different environments - that suggests a strong genetic contribution.

• Family analysis: Tracking how often traits (such as a predisposition to depression or high achievement) appear among relatives can suggest genetic patterns. However, because family members usually share both DNA and environment, these findings are informative but not conclusive.

• Adoption studies: If people raised by non-biological families resemble their biological relatives more than their adoptive relatives on certain traits, genetics is likely a major influence. If they resemble their adoptive relatives more, that points to a stronger environmental effect.

Overall, findings show that neither genetics nor experience works alone. Traits such as intelligence show moderate heritability, yet education quality and nutrition can strongly shape development. Behavior reflects ongoing interaction: genes can influence how responsive someone is to an environment, and environments can influence how genes are expressed.

Example: Suppose researchers find that identical twin pairs have a correlation of .75 on a measure of anxiety, while fraternal twin pairs show a correlation of .35. What does this pattern suggest?

Because identical twins share essentially all of their DNA while fraternal twins share about half, the much larger similarity among identical twins points to a meaningful genetic contribution to anxiety. If the same pattern held for identical twins raised apart - still showing high similarity despite different environments - that would strengthen the genetic interpretation even further. At the same time, the fact that identical twins aren’t perfectly correlated (1.0) reminds us that environment still plays a role.

Pitfall: heritability ≠ genetic destiny

A heritability estimate tells you how much of the variation in a trait within a population is linked to genetic differences - it does not mean the trait is fixed or inevitable. High heritability still leaves room for environmental influences to make a real difference. Also, remember that twin and adoption studies show correlation, not causation: shared genes are associated with similar traits, but we can’t conclude that those specific genes directly cause the trait.

Nervous system

Genetics provides a foundation of possibilities, and experience can strengthen, weaken, or redirect those possibilities. The system that turns these influences into actions, sensations, and memories is the nervous system. The nervous system is an intricately connected network of tissues and cells that supports awareness, action, and adaptation. It has two main parts: the central nervous system and the peripheral nervous system.

Sidenote

Neuron basics

The basic building block of the nervous system is the neuron - a specialized cell that transmits electrical and chemical signals. Neurons communicate across tiny gaps called synapses, releasing chemical messengers known as neurotransmitters. These processes are covered in detail in the biological bases chapter.

Central nervous system (CNS)

The CNS includes the brain and spinal cord. It acts as the main control center for integrating sensory information and directing complex, intentional actions.

The brain interprets perceptions, organizes memories, forms plans, and controls both voluntary and involuntary activity. Different brain regions have distinct (but connected) roles. For example, decision-making, social judgment, and self-regulation rely heavily on the frontal lobe, while the occipital lobe (toward the back of the head) specializes in processing visual information.

The spinal cord, which extends downward from the brain, serves as both a communication pathway and a control center for protective reactions. It carries information quickly between the brain and the body. Certain protective movements - called reflex arcs - route signals through the spinal cord without first traveling to the brain, which is why your hand pulls away from a painful stimulus before you’re consciously aware of the injury.

Peripheral nervous system (PNS)

The PNS consists of neural tissue that branches out from the CNS. It includes systems that control deliberate actions (like moving your fingers to type or your feet to run) and systems that regulate automatic functions that keep you alive (such as breathing and heartbeat). The PNS has 2 subsystems: somatic and autonomic.

Somatic subsystem: This system controls voluntary movement. Motor fibers send impulses to muscles (allowing actions such as speaking, writing, or running). It also carries sensory information from the environment (such as hot, cold, rough, or smooth), which helps guide your next actions.

Autonomic subsystem: This system operates largely outside awareness. It has two complementary divisions (sympathetic and parasympathetic). Together, these divisions help the body respond to demands while maintaining internal stability.

• Sympathetic division: Activates quickly in emergencies, preparing the body to face danger or escape (such as increasing heart rate and sharpening attention). This division is commonly called “fight or flight.”

• Parasympathetic division: Promotes calm after a threat has passed and supports processes such as slowing the pulse, aiding digestion, and conserving energy. This division is commonly called “rest and digest.”

Pitfall: sympathetic vs. parasympathetic

A common mistake is mixing these two up on exam questions. A helpful memory cue: the S ympathetic system handles S tress (fight or flight), while the P arasympathetic system handles P eace (rest and digest). They work as opposites: what one speeds up, the other slows down.

What are the two main divisions of the autonomic subsystem in the peripheral nervous system, and what are their roles?

When the system falters

Health and well-being depend on smooth functioning across this network. Injury to the CNS can affect thinking, memory, or movement. Disruptions in the autonomic system can contribute to chronic stress or problems with essential body rhythms. Damage to somatic components can reduce the precision or strength of movement. Some disorders that affect memory, reasoning, or the ability to regulate fear reflect disruptions in neural circuits shaped by a combination of genes and experience.